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unsilvered part of the horizon glass fall before the object glass of the telescope, when this is necessary.

When the sun is the body observed, the light is generally too intense for the eye to bear with the telescope; to render the rays tolerable to the sight, the coloured glasses above mentioned are interposed, and the blank tube instead of the telescope is applied, by which means the sun may be viewed without inconvenience.

The adjustments required in the sextant are the following:The index and horizon glasses must be perpendicular to the plane of the instrument.

The planes of these glasses must be parallel to each other when the zero of the vernier is set to zero on the limb.

The optical axis of the telescope must be parallel to the plane of the instrument.

To test the perpendicularity of the index glass.

Slide the index to about 40° or 45° on the limb, and holding the instrument nearly parallel to the horizon, that is, about level, look obliquely into the index glass to see whether the sharp edge of the limb seen by reflection is an exact and straight continuation of the same edge of the limb seen by direct vision. If the edge and its image form one continued arc, the index glass is perpendicular to the plane of the instrument; but if otherwise, the instrument must be sent to the maker to adjust.

To adjust the horizon glass.

Fix the zero of the vernier to zero on the limb, hold the instrument level, and looking through the eye end steadily at some convenient object, sweep the index slowly along the limb; if the reflected image do not pass so as to coincide with the direct object, but the one projects beyond the other, then the glasses are not perpendicular to the plane of the sextant, and supposing we have already tested the index glass, then the horizon glass is at fault; this is adjusted by turning a small screw at the bottom of the frame in which it is set until the reflected image passes exactly over the direct object; the horizon, or any straight distant line, may be used for this

purpose.

To examine the parallelism of the planes of the two glasses when the index is at zero, or to determine the index error.

The best way to do this is to measure the sun's diameter both to the left and to the right of zero; that is, first on the arc of

the limb, properly so called, and next on the arc of excess. First clamp the index at about 30' to the left of zero, and with the sun for object, bring, by means of the tangent screw, the reflected image of the upper limb into contact with the direct view of the lower limb, and set down the minutes and seconds marked by the vernier. Now clamp the index at about 30′ on the arc of excess to the right of zero, and bring the reflected image of the lower limb into contact with the direct image of the upper limb, and take off the reading from the vernier. Set down this reading beneath the last observed; then half the sum of the two is the correct diameter of the sun, and half their difference is the index error. When the reading on the arc of excess is the greater of the two, then the index error must be added to all the readings made with the instrument; when less, then the index error must be subtracted; it is as well to repeat this two or three times, and the difference of the sums of the readings on the arc, and on the arc of excess divided by the number of times the operation has been repeated, will be the index error; and the sum of all the readings divided by the number will be the sun's diameter.

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To examine the optical axis or line of collimation. When the sun and moon, having a distance of 90° or more, brought into contact just at the wire of the telescope nearest to the plane of the instrument, the index fixed, and the position of the instrument altered to make the objects appear on the other wire, then if the contact still remains perfect, the axis of the telescope is in proper adjustment; but if otherwise, alter it by means of the two screws which fasten the collar into which the telescope screws to the up and down piece, tightening one screw and turning back the other, till after a few trials the contact remains perfect at both wires.

To examine the error from the imperfection of the dark glasses.

With the dark glass only at the eye end of the telescope, first bring the reflected image of the sun into contact with his image seen directly through the unsilvered half of the horizon glass. In the next place, having removed this dark glass, set up separately, and then in combination, the various coloured glasses; the angle through which the index must be moved to perfect the contact is the error of the dark glasses, or of any combination of them, used in making the observations. This error should be recorded for each glass, and for each combination of glasses, and so as to be easily referred to.

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To examine the limb and the vernier.

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When all these observations and adjustments have been made, the instrument may be applied in practice, if we may rely on the skill and reputation of the maker; but if otherwise, it is as well to examine the materials of which the instrument is made. The accuracy of the divisions on the limb and vernier may be tested by sliding the latter slowly along the former, and examining how they agree, or as it were stepping the arc of the limb with the vernier, remembering what we have said about the total length and the divisions of the latter in another place, under the head of "Vernier;" the intermediate strokes on the vernier itself may be examined, lastly, by the strokes on the limb taken in various places. Of course, in doing this, the microscope is to be used; the divisions should all be fine and distinct, and the inlaid plates, on which they are engraved, should be perfectly level with the surface of the instrument. If all this be not found perfectly correct, the instrument should be rejected, or at least fresh divided by a good maker.

To examine the glasses of the mirrors: first, to try the quality of the glass, whether the surfaces be perfect planes, and whether the faces of each glass are parallel.

Bring two distinct and distant objects into good contact, and let them be seen at the upper edge of the silvered part; now move the instrument in its own plane, and move the image and body along the line of separation or edge of the silvered part. If the coincidence is not disturbed, the plane is perfect. Or look separately into each reflector in an oblique direction, and observe the image of some object; if it appears clear and distinct, the glass is of good quality, but if it appears notched or irregular, the glass is bad. Again, if the image appears singly and welldefined about the edges, then the two surfaces of the glass are parallel, but if misty, or appearing like two images, the two faces of the glass are not parallel, and consequently it is not fit for its purpose. This examination may be performed with the flame of a candle.

Very possibly, on reading these instructions as to the examination of all the parts successively of the sextant, they will be considered very tedious, but it must be remembered that when they are once performed the work will not have to be repeated if the instrument is taken proper care of; in the next place, nothing will tend more to make the reader thoroughly acquainted with the principle and construction of the instrument; to this we must add, that it is not one of ordinary use, being principally

employed for astronomical observation, where these are required,. for maritime surveying, and a few more important matters. A little attention to the nature and construction of the instrument will show of what it is capable; but there being no outward case to protect the various parts, it will not bear the slightest approach at ill usage.

When an observation is to be made with the sextant, it is evident that the plane of the instrument must be held in the same place as the two objects of which the angle is required; vertically, therefore, if altitudes are to be taken, and horizontally or oblique where horizontal or oblique angles are to be taken. The reader will at first find some little difficulty in thus adjusting the plane of the instrument, and his first attempts will very likely be failures, but as, indeed, in the use of all instruments, after a few further trials, facility will gradually come round. The instrument is to be held in the right hand by the handle underneath, though this will, of course, be sideways when the plane of the instrument is held vertically; and it should not be grasped tight, but held with ease, though with safety, for by holding it too hard the hand is apt to grow unsteady.

Supposing now an altitude of an object-as of the sun, for instance-is to be taken, with the horizon of the sea before the observer; one or two of the dark glasses must be used according to the brilliancy of the sun; the eye is applied to the eye-piece, and the sight directed to that part of the horizon immediately beneath the sun, and, at the same time, the left hand gently glides the index forward, until the image of the sun, reflected from the index glass, appears in contact with the horizon at the line of demarcation between the silvered and unsilvered portions of the horizon glass; clamp the index, and with a gentle motion of the tangent screw, perfect the contact of the upper or lower limb (or part) of the sun with the horizon, when it will appear a tangent to his circular disc. To the angle read off apply the index error (see ante); then add or subtract the sun's semi-diameter,‡ as given in the Nautical Almanac, ac

The altitude or elevation of any point of the sphere is an arc of a vertical circle intercepted between it and the sensible horizon; this is peculiarly denominated sensible or apparent horizon, to distinguish it from the rational or true horizon; the higher the eye the further is the visible horizon extended.

If the observer knows his latitude approximately he may find the miridional latitude nearly, to which he may at once set his instrument: he will then have only a small quantity to move his index to perfect the observation. Take from the Nautical Almanac the declination of the object, and if it be the same name with the latitude, add it to the co-latitude; if of a different name, subtract it; the sum or difference will be the meridian latitude.

The apparent diameter of the sun or moon is the angle under which they appear to the observer; the amount of this depends upon the real magnitude

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cording as the lower or upper limb has been observed to obtain the apparent altitude of the sun's centre. Before, however, this observation can be used for determining the time, the latitude, &c., it must be further corrected for parallax and refraction,† to obtain the true altitude, adding the former, and deducting the latter. A quantity must also be deducted for the dip when the sea horizon is used, which is not the case when an artificial horizon is employed; but this will be mentioned hereafter.

When the angle subtended by any two objects more or less in an horizontal plane is to be measured, it is best to direct the of the object and its distance from the observer. The sun's semidiameter is entered in the Nautical Almanac. Its semi-diameter is 16' which is often used in practice, as it does not vary more than half a minute from that amount.

The situation of a celestial body as viewed from the surface of the earth, is called its apparent place, and that part of the heavens where it would be viewed, if observed at the same moment from the centre of the earth, is called its true place. The difference between the two is called the parallax of the object. This parallax is greatest at the horizon, gradually diminishes as it rises above the horizon, and vanishes when the object reaches the zenith. As the altitude of an object, as observed from the earth's surface, is less than if observed from the centre, the parallax is to be added to the apparent altitude to obtain the true altitude.

The rays of light from a celestial body on entering the atmosphere obliquely are diverted from their rectilinear course, and are bent more and more towards the centre of the earth as they dive deeper into the atmosphere; hence they fall upon the eye in a different direction from that of the object, and make it appear higher than its real place. The difference thus caused between the object's real and apparent places is called the refraction. The more obliquely the rays fall, the greater the diversion from their rectilinear course, and the greater the refraction; therefore it is greatest at the horizon, and vanishes at the zenith. It is always to be deducted from an object's apparent altitude, because the effect of refraction is to make bodies appear higher than they really are; so much so, that celestial objects may appear above the horizon when they are actually below it. Tables of refraction are usually found in nautical works.